meta-phosphor: phosphor-ipmi-fru: fix LICENSE warning

Bitbake currently gives an `obsolute-license` warning for any
of the recipes using `phosphor-ipmi-fru.inc`.  Fix this by switching
the LICENSE variable to `GPL-3.0-or-later`, which matches the contents
of the LICENSE file in the repository.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ife34a6bfaf564b530a889a3fa277ed84c0f2a640